M1 L3: Linkage and Chromosomes

• Mendelʼs “hereditary factorsˮ were purely abstract when first proposed
• Today, we can show that the factors—genes—are located on chromosomes
• Locating genes along chromosomes

  Tagging isolated chromosomes with a fluorescent dye that highlights the gene
• Cytologists worked out the process of mitosis

  1875
• Cytologists worked out the process of meiosis

  1890s
• Biologists began to see parallels between the behavior of Mendelʼs proposed hereditary factors and chromosomes
• Sutton and Boveri and others independently noted parallels between Mendelʼs factors and chromosomes and began to develop the chromosome theory of inheritance


  1902
• For his work, Morgan chose to study Drosophila melanogaster, a common species of fruit fly
• Characteristics of fruit flies for genetic studies

  • Produce many offspring
  • A generation can be bred every two weeks
  • Have only four pairs of chromosomes
• Morgan noted wild / normal type, phenotypes that were common in the fly populations
• Traits alternative to the wild type are called mutant phenotypes
• The first mutant Morgan discovered was a fly with white eyes instead of the wild-type red eyes
• Correlating behavior of a gene's alleles with behavior of chromosome pair

  1. Morgan mated male flies with white eyes with female flies with red eyes

  2. F1 generation all had red eyes

  3. F2 generation showed a 3:1 red to white eye ratio, but only males had white eyes

  4. Morgan reasoned that the white-eyed mutant allele must be located on the X chromosome

  5. Morgan's findings supported the chromosome theory of inheritance
• Humans and other mammals have two types of sex chromosomes: a larger X chromosome and a smaller Y chromosome
• A gene on the Y chromosome called SRY is responsible for development of the testes in an embryo
• A gene that is located on either sex chromosome is called a sex-linked gene
• Genes on the Y chromosome are called Y-linked genes
• Only 78 genes, coding for about 25 proteins, have been identified on the human Y chromosome
• Genes on the X chromosome are called X-linked genes; the human X chromosome contains about 1.1k genes
• 1. linked genes follow a specific pattern of inheritance
• For a recessive X-linked trait to be expressed: a female needs 2 copies of the allele (homozygous), a male needs only one copy of the allele (hemizygous)
• X linked recessive disorders are much more common in males than in females
• Some disorders caused by recessive alleles on the X chromosome in humans: Color blindness (mostly X-linked), Duchenne muscular dystrophy, Hemophilia
• In mammalian females, one of the two chromosomes in each cell is randomly inactivated during embryonic development
• The inactive X chromosome condenses into a Barr body
• If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character (combination)
• Locating Genes along Chromosomes

  1. Mendelʼs “hereditary factorsˮ were purely abstract when first proposed

  2. Today, we can show that the factors-genes are located on chromosomes

  3. The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

  4. Cytologists worked out the process of mitosis in 1875 and meiosis in 1890s using improved techniques of microscopy

  5. Biologists began to see parallels between the behavior of Mendelʼs proposed hereditary factors and chromosomes

  6. Around 1902, Sutton and Boven and others independently noted these parallels and began to develop the chromosome theory of inheritance
• Fruit flies as a convenient organism for genetic studies

  • They produce many offspring
  • A generation can be bred every two weeks
  • They have only four pairs of chromosomes
  • Wild type, or normal, phenotypes were common in the fly populations
  • Traits alternative to the wild type are called mutant phenotypes
  • The first mutant Morgan discovered was a fly with white eyes instead of wild-type red eyes
• Pedigree Analysis


  1. A pedigree is a family tree that describes the interrelationships of parents and children across generations

  2. Inheritance patterns of particular traits can be traced and described using pedigree

  3. Pedigrees can also be used to make predictions about future offspring

  4. Common symbols in reading a pedigree chart
• 1. linked Recessive Inheritance


  • Females who are homozygous and males who are homozygous express the phenotype
  • An affected mother passes the trait to all the offspring but only the son exhibits the phenotypes
  • An affected father passes the trait to all of his daughters but to none of his sons. In this case, the daughters will only be carriers
• 1. linked Dominance Inheritance

  • Individuals who have the dominant allele on the X chromosomes exhibit the phenotype
  • An affected male passes the allele to all daughters but to none of his sons
  • An affected female passes the alleles to all the offspring unless she is a heterozygote
• Linked genes tend to be inherited together


  • Because they are located near each other on the same chromosome
  • Genes that are located on the same chromosome tend to be inherited together and are called linked genes
• Nonparental phenotypes in testcross


  This involves genetic recombination, the production of offspring with combinations of traits differing from either parent
• Alterations of chromosome number or structure cause genetic disorders

  • Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
  • Plants tolerate such genetic changes better than animals do
• In nondisjunction, pairs of homologous chromosomes do not separate normally during cell division
• Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
• Plants tolerate such genetic changes better than animals do
• Nondisjunction

  1. Pairs of homologous chromosomes do not separate normally during meiosis

  2. One gamete receives two of the same type of chromosome, and another gamete receives no copy
• Aneuploidy results from the fertilization of gametes in which nondisjunction occurred
• Offspring with aneuploidy have an abnormal number of a particular chromosome